The Second World War veteran, whose fundraising for the NHS hit £31m on his birthday, said he had been overwhelmed by the generosity of the public and signed off his letter of thanks with the words: “Please always remember, ‘Tomorrow Will Be A Good Day’.”

The nation has joined in marking the centenarian’s milestone birthday, with Boris Johnson joining in wishing him many happy returns.

In a video message posted on his Twitter account, the prime minister said: “Your heroic efforts have lifted the spirits of the entire nation” and described him as: “a point of light in all our lives.”

Within days that amount had soared to tens of millions of pounds and hit £30m on his birthday.

Captain Tom Moore waves to a Battle of Britain Memorial Flight flypast of a Spitfire and a Hurricane passing over his home as he celebrates his 100th birthday. (PA)

A Battle of Britain Memorial Flight flypast of a Spitfire and a Hurricane to mark Capt Moore's 100th birthday. (PA)

On Thursday, he was honoured with an RAF flypast including a Spitfire and was also appointed as an honorary colonel of the Army Foundation College. The veteran has also awarded the Freedom of the City of London in recognition of his charity work.

His popularity has made his 100th birthday an occasion marked by people across the country, with so many people sending him cards to mark it that a dedicated sorting office was set up at his grandson’s school.

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Royal Mail has also honoured Capt Moore with a special postmark and by painting a postbox in NHS blue in tribute to his fundraising efforts.

The postbox, in his home village of Marston Moretaine, Bedfordshire, has been decorated with a golden balloon.

Bill Chandi, who has been Marston Moretaine’s postmaster for 32 years, said: “This has been an incredible time – far, far busier than even Christmas, but I am very happy to help as Captain Tom is a remarkable man, a real role model.”

In a message to supporters, Capt Moore said: “Reaching 100 is quite something. Reaching 100 with such interest in me and huge generosity from the public is very overwhelming.

A man polishes a letter box painted blue to mark the 100th birthday of Capt Tom Moore close his home in Marston Moretaine, near Bedford. (PA)

“The donations page will close at midnight this evening. NHS Charities Together still have their urgent appeal, so people can donate to them that way.”

He said he planned to spend his birthday with his family then would be having a few days’ rest.

“My legs may be tired, but my mind is racing and I’m hoping to be back very soon with other ways in which I can help people, help others.

“Please always remember, ‘Tomorrow Will Be A Good Day’. With my sincerest gratitude, Captain Tom Moore.”

Speaking to Good Morning Britain on Thursday, he said people’s generosity in donating had been “amazing”.

“We started off with such a little figure and we did that with hope and look what we’ve got today, such a fabulous sum for the NHS… It’s difficult to say how I feel about it because it’s such a magnificent thing these people have done throughout the world to provide this sum of money.”

He added: “To all the people of Britain, thank you very, very much. It just shows what a marvellous country we are… We all know we’re going to pull through. It may be difficult, it may take time but we will win through in the end.” (edited)

Capt Moore was born in Keighley, West Yorkshire, on 30 April 1920 and went on to serve in the Second World War, fighting in western Burma.

After the war, he returned to the UK and worked as an instructor at the Armoured Fighting Vehicle School in Bovington, Dorset.

A man delivers a gift to the home of Captain Tom Moore in Bedford as he celebrates his 100th birthday. (PA)

Capt Moore's daughter Hannah Ingram-Moore, centre, and his grandchildren Benjie and Georgia Ingram-Moore greet people outside his home in Bedford. (PA)

He suffered a broken hip in 2018, which his family said inspired him to want to help the NHS.

His fundraising efforts have attracted the backing of celebrities, politicians and royals, including Sir Mo Farah, Lewis Hamilton and Gary Lineker, health secretary Matt Hancock and the Duke of Sussex.